1. Field of the Invention
This invention relates to a method for aberration-free optical recording of highly resolved sonar or radar pictures or maps the term maps being used herein, wherein a series of one-dimensional holograms is recorded as a function of the receiving time through the use of pulsed sonar or radar and are coherently optically reconstructed.
2. Description of the Prior Art
Highly resolved images can be obtained by means of ultrasonic signals with the help of a sonar method, and in particular a side-looking or lateral-view sonar method. In this method, a succession of sonar pulses is transmitted from a vehicle, such as a ship. The echoes of the targets impinged upon by the radiation are received by sound transducers which are arranged parallel to the direction of travel of the vehicle. After a reference signal has been superimposed, which reference signal is coherent with respect to the transmitter, the signals are recorded in such a way that the receiving time is recorded in one coordinate direction while the object coordinate is contained parallel to the vehicle direction in the form of one-dimensional holograms which are aligned with respect to phase. The same principle is also true in connection with other waves, such as high frequency electromagnetic waves, as they are applied, for example, in radar techniques.
Highly resolved sonar or radar maps can be produced by means of coherent optical reconstruction of such one-dimensional holograms and subsequent recording of the object images produced thereby. Since, however, the image distance depends on the object distance during the reproduction of the holograms, the two object coordinates, distance and azimuth, are focused in different planes, whereby the reconstruction plane of the azimuth coordinate extends at an angle to the optical axis. Therefore, a conical lens, as well as a cylindrical lens, is required for the recordation of the object. As the ratio of image distance/object distance approaches the order of magnitude of unity, the required conical lens can be no longer be realized without disadvantageous aberrations.
Furthermore, the distance-dependent object enlargement will cause strong image distortions with the above-mentioned type of recording. The film upon which the one-dimensional radar holograms are recorded can be moved along a conical lens at a constant speed with lateral view radar for the correction of these image distortions, and the radar map can be simultaneously recorded line by line upon a film which is also pulled along behind a slot at a constant speed. However, this method has the drawback that the radar map is recorded line by line, so that the radar image can only by viewed after the holograms have been completely pulled along the conical lens and the film carrying the image has been developed. Furthermore, the slot causes undesirable defraction effects which degrade the resolution.
In order to record the entire sonar or radar map completely and without delay in such a way that the image enlargements and the image distances are independent of object distance, it was suggested in my copending application, Ser. No. 342,302, filed Mar. 12, 1973 now U.S. Pat. No. 3,895,341, to effect a one-dimensional cross correlation of the transmission function of the holograms through the use of a filter which is adapted in accordance with the Fourier transform function of a conical lens. I have found, however, that this method can be improved upon by eliminating the additional component, the adapted filter, and accordingly eliminate the essential production costs due to the defined phase transmission of the element, in addition to the adjustment requirements thereof.